BaseTools: 64bit FSP FV map file cannot be created correctly

BaseTools: 64bit FSP FV map file cannot be created correctly

https://bugzilla.tianocore.org/show_bug.cgi?id=4035
64bit FSP FV map file cannot be created correctly when using CLANG
compiler. When compiling 64bit FSP with CLANG, there is no prefix
symbol '_' added to function name. Hence FSP FV map file cannot be
created properly. Updated the if condition for CLANG in GenFv to
get the issue fixed.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Cc: Lee Hamel <lee.m.hamel@intel.com>
Signed-off-by: Ted Kuo <ted.kuo@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>