tccasm: Implement .set sym, expr

That, as well as "sym = expr", if expr contains symbols.
Slightly tricky because a definition from .set is overridable,
whereas proper definitions aren't.

This doesn't yet allow using this for override tricks from C
and global asm blocks because the symbol tables from C and asm
are separate.

tests/asmtest.S

@@ -824,6 +824,17 @@ nop
 .skip (-((4b-3b) > 0) * 2) , 0x90
 .popsection
 
+.globl overrideme
+.weak overrideme
+  nop
+.globl notimplemented
+notimplemented:
+  ret
+.set overrideme, notimplemented
+overrideme = notimplemented
+overrideme:
+  ret
+
     movd %esi, %mm1
     movd %edi, %xmm2
     movd (%ebx), %mm3