Fix x86 ClangCL and GCC/Clang -std=c## compiles

[p0lymeric]

Checklist

• if this fixes something: added a Fixes: tag to the commit message

Fixes compile for some x86 compiler configurations, including ClangCL on Windows, and GCC/Clang in Standard C mode (-std=c##).

Fixes #767

Notes:

• Reintroduced some nasty-looking parts from this past commit (aes_desc.c:L66-78):
  • ec3804c#diff-4b49c40dd5787d702d14e1859464a5941919a8dd8288dce824014f07939e321eL66-L78
• ClangCL is handled by falling through defined(_MSC_VER) && !defined(__clang__) into the GNU-style inline asm block
• __asm__ instead of asm so that gcc/clang accept the inline asm block in Standard C mode

[karel-m]

there already exists a similar patch e9a7e94 in #755