i386: Remove unused variable in sysdeps/x86/cacheinfo.c

Commit a98dc92d ("x86: Add cache
information support for Zhaoxin processors") introduced an unused
variable warning in the default i686-linux-gnu build:

In file included from ../sysdeps/i386/cacheinfo.c:3:
../sysdeps/x86/cacheinfo.c: In function 'init_cacheinfo':
../sysdeps/x86/cacheinfo.c:762:16: error: unused variable 'eax' [-Werror=unused-variable]
  762 |   unsigned int eax;
      |                ^~~