crt: Manually do the linker test for __ImageBase

Instead of using AC_LINK_IFELSE, construct a test file and manually
run a compile/linking command.

This fixes the linker test when bootstrapping a toolchain from
scratch.

This matches the previous configure time link test we had, which
was removed in 6f53dbf6d423fab5a9fc6b3e4b07bdcf66d253b5. See
baaf945d53f7dedb15b6f5342bd04e8fd4952211,
30af18252d4e965e98612e215b8cf6b7ae42c01a,
ccda8b5229e428a6fa7036140778b23d11b554e8 and
bda8fc0064e12d07e0f0e0c696dc0568b13c096f for earlier attempts at
making configure time linker tests that work across various tools
when bootstrapping the toolchain itself; this solution incorporates
all the lessons learnt from the previous linker tests.

Signed-off-by: Martin Storsjö <martin@martin.st>