backported setjmp/longjmp changes from v7.x branch:

Hand-picked from commits 02390fa9c1, 436ad4b830, and 844cb490ab:
- Only use _setjmp3 on i386.  Don't use _WIN64 to decide this, but
  check explicitly for i386, and use _setjmp on other platforms.
- setjmp.h: Only pass a frame pointer to setjmp if SEH is enabled
  This should avoid the need for clearing the frame pointer in the
  longjmp wrapper.
- crt: Use importlibs for longjmp.

Fixes setjmp/longjmp code built with -fno-asynchronous-unwind-tables
with SEH toolchains.

mingw-w64-crt/def-include/msvcrt-common.def.in

diff --git a/mingw-w64-crt/def-include/msvcrt-common.def.in b/mingw-w64-crt/def-include/msvcrt-common.def.in
index 76e1fa3..a27fd22 100644
--- a/mingw-w64-crt/def-include/msvcrt-common.def.in
+++ b/mingw-w64-crt/def-include/msvcrt-common.def.in
@@ -125,11 +125,11 @@
 ;logb
 ADD_UNDERSCORE(nextafter)
 
+longjmp
+
 _daylight DATA
 _timezone DATA
 _tzname DATA
 ADD_UNDERSCORE(daylight)
 ADD_UNDERSCORE(timezone)
 ADD_UNDERSCORE(tzname)
-
-longjmp DATA

mingw-w64-crt/lib32/crtdll.def

diff --git a/mingw-w64-crt/lib32/crtdll.def b/mingw-w64-crt/lib32/crtdll.def
index 1c73001..38e300a 100644
--- a/mingw-w64-crt/lib32/crtdll.def
+++ b/mingw-w64-crt/lib32/crtdll.def
@@ -598,7 +598,7 @@
 ;_localtime32 = localtime
 log
 log10
-longjmp DATA
+longjmp
 malloc
 mblen
 mbstowcs

mingw-w64-crt/lib64/ntdll.def

diff --git a/mingw-w64-crt/lib64/ntdll.def b/mingw-w64-crt/lib64/ntdll.def
index cc81a06..d08f498 100644
--- a/mingw-w64-crt/lib64/ntdll.def
+++ b/mingw-w64-crt/lib64/ntdll.def
@@ -1963,7 +1963,7 @@
 isxdigit
 labs
 log
-longjmp DATA
+longjmp
 mbstowcs
 memchr
 memcmp

mingw-w64-crt/misc/mingw_getsp.S

diff --git a/mingw-w64-crt/misc/mingw_getsp.S b/mingw-w64-crt/misc/mingw_getsp.S
index 3c99658..7992abd 100644
--- a/mingw-w64-crt/misc/mingw_getsp.S
+++ b/mingw-w64-crt/misc/mingw_getsp.S
@@ -25,26 +25,3 @@
 	mov	r0, sp
 	bx	lr
 #endif
-
-/* On ARM:
- * Error: cannot represent BFD_RELOC_32_PCREL relocation in this object file format
- * But anyway, nothing is needed here as libarm32/libmsvcrt.a is exporting longjmp
-  ldr ip, 1f
-  ldr pc, [pc, ip]
-  1: .long __imp_longjmp - (1b + 4)
-*/
-#if !(defined(_ARM_) || defined(__arm__))
-	.globl __MINGW_USYMBOL(longjmp)
-	.def	__MINGW_USYMBOL(longjmp);	.scl	2;	.type	32;	.endef
-__MINGW_USYMBOL(longjmp):
-#if defined(_AMD64_) || defined(__x86_64__)
-#ifndef __SEH__
-  xorq %rax,%rax
-  movq %rax, (%rcx)
-#endif
-  leaq __MINGW_IMP_LSYMBOL(longjmp)(%rip), %rax
-  jmpq *(%rax)
-#elif defined(_X86_) || defined(__i386__)
-  jmp *__imp__longjmp
-#endif
-#endif /* !(defined(_ARM_) || defined(__arm__)) */

mingw-w64-headers/crt/setjmp.h

diff --git a/mingw-w64-headers/crt/setjmp.h b/mingw-w64-headers/crt/setjmp.h
index 5be566d..d116bd4 100644
--- a/mingw-w64-headers/crt/setjmp.h
+++ b/mingw-w64-headers/crt/setjmp.h
@@ -169,24 +169,28 @@
 #define _JMP_BUF_DEFINED
 #endif
 
+_CRTIMP __MINGW_ATTRIB_NORETURN __attribute__ ((__nothrow__)) void __cdecl longjmp(jmp_buf _Buf,int _Value);
+
 void * __cdecl __attribute__ ((__nothrow__)) mingw_getsp (void);
 
 #ifndef USE_NO_MINGW_SETJMP_TWO_ARGS
 #  ifndef _INC_SETJMPEX
-#    ifdef _WIN64
+#    if defined(_X86_) || defined(__i386__)
+#      define setjmp(BUF) _setjmp3((BUF), NULL)
+#    elif defined(__SEH__)
 #     if (__MINGW_GCC_VERSION < 40702)
 #      define setjmp(BUF) _setjmp((BUF), mingw_getsp())
 #     else
 #      define setjmp(BUF) _setjmp((BUF), __builtin_frame_address (0))
 #     endif
 #    else
-#      define setjmp(BUF) _setjmp3((BUF), NULL)
+#     define setjmp(BUF) _setjmp((BUF), NULL)
 #    endif
   int __cdecl __attribute__ ((__nothrow__,__returns_twice__)) _setjmp(jmp_buf _Buf, void *_Ctx);
   int __cdecl __attribute__ ((__nothrow__,__returns_twice__)) _setjmp3(jmp_buf _Buf, void *_Ctx);
 #  else
 #    undef setjmp
-#    ifdef _WIN64
+#    ifdef __SEH__
 #     if (__MINGW_GCC_VERSION < 40702)
 #      define setjmp(BUF) _setjmpex((BUF), mingw_getsp())
 #      define setjmpex(BUF) _setjmpex((BUF), mingw_getsp())
@@ -210,9 +214,6 @@
   int __cdecl __attribute__ ((__nothrow__,__returns_twice__)) setjmp(jmp_buf _Buf);
 #endif
 
-  __declspec(noreturn) __attribute__ ((__nothrow__)) void __cdecl ms_longjmp(jmp_buf _Buf,int _Value)/* throw(...)*/;
-  __declspec(noreturn) __attribute__ ((__nothrow__)) void __cdecl longjmp(jmp_buf _Buf,int _Value);
-
 #ifdef __cplusplus
 }
 #endif