Enable macro test for clang compiler

`clang` uses `-fdebug-macro` switch to enable debug-info
for macros.

gdb/testsuite/ChangeLog:

2021-03-19  Sourabh Singh Tomar  <SourabhSingh.Tomar@amd.com>

* gdb.base/info-macros.exp: Append -fdebug-macro to
  additional_flags for clang.
* gdb.base/macscp.exp: Likewise.
* gdb.base/style.exp: Likewise.
* gdb.linespec/macro-relative.exp: Likewise.