BaseTools/Conf: Add new macro for customizing dll file reduction.

BaseTools/Conf: Add new macro for customizing dll file reduction.

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3810

New macro OBJCOPY_STRIPFLAG is added in build_rule.template to replace
'--strip-unneeded -R .eh_frame', so that module can have some unique
objcopy flags for its own purpose.
In tools_def.template, set '--strip-unneeded -R .eh_frame' as default
value of OBJCOPY_STRIPFLAG.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Signed-off-by: Wei6 Xu <wei6.xu@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>