x86 and amd64 tests: Use .text and .previous around all top-level asm.

x86 and amd64 tests: Use .text and .previous around all top-level asm.

GCC10 defaults to -fno-common which exposes some latent bugs in
some of the top-level asm code in various .c test files. Some of the
tests started to segfault (even if not run under valgrind). Such code
needs to be wrapped inside a .text and a .previous asm statement to
make sure the code is generated in the .text code section and to
make sure the compiler doesn't lose track of the section currently
being used to generate data or code in. Without it code might be
generated inside a data section or the other way around.