drop alloca #define

(Because GNU's alloca.h unconditionally #undef's alloca)

Also, remove gcc specific sections in headers. and
instead change tests such that gcc does not use them.

lib/alloca86-bt.S

@@ -3,9 +3,9 @@
 
 #include "../config.h"
 
-.globl __bound__alloca
+.globl __bound_alloca
 
-__bound__alloca:
+__bound_alloca:
     pop     %edx
     pop     %eax
     mov     %eax, %ecx

lib/alloca86.S

@@ -3,9 +3,9 @@
 
 #include "../config.h"
 
-.globl _alloca
+.globl alloca
 
-_alloca:
+alloca:
     pop     %edx
     pop     %eax
     add     $3,%eax