patch 9.1.0622: MS-Windows: mingw-build can be optimized

patch 9.1.0622: MS-Windows: mingw-build can be optimized

Problem:  MS-Windows: mingw-build can be optimized
Solution: use --gc-sections to reduce the size of the executable
          (Ken Takata)

Use the --gc-sections linker option and the -ffunction-sections compiler
option to reduce the size of the executable files.  To make these work,
the -fno-asynchronous-unwind-tables compiler option is also needed.

This is enabled by default and can be disabled by `USE_GC_SECTIONS=no`.

Note: A similar feature has been already used in MSVC. (The /OPT linker
option.)

related: #15350

Signed-off-by: Ken Takata <kentkt@csc.jp>
Signed-off-by: Christian Brabandt <cb@256bit.org>