Allow TARGET_LDFLAGS setting by T_A

[Ralph Lange]

On Windows:
MinGW and MSVC share the OS_CLASS of "WIN32".
LDFLAGS is the only FLAGS setting that - when set for a specific make target - doesn't allow configuring by target architecture, only by OS_CLASS.
I.e., setting
    myapp_CFLAGS_windows-x64
works, while trying to do the same with
    myapp_LDFLAGS_windows-x64
does not work.

Problem: A specific test program (epicsStackTracetest) needs "-debug" in the LDFLAGS, but only for the Microsoft compiler, not for GCC based MinGW.

Note: this change will need matching changes in the documentation (AppDevGuide, chapter on the build system).

[Ralph Lange (ralph-lange)]

Attaching a patch for this.

[Ralph Lange (ralph-lange)]

Patch had side effects - working on an update.

[Ralph Lange (ralph-lange)]

Invalid.

There are two mechanisms:
1. _<T_A> then _<OS_CLASS> then <no suffix>, cumulative
2. _<OS_CLASS> if defined, else _DEFAULT, alternative

1. is used for INCLUDES CFLAGS CXXFLAGS CPPFLAGS
2. is used for SRCS RCS ...OBJS ...LIBS and LDFLAGS

The use case calls for a cumulative T_A element for LDFLAGS, which is breaking the design.

Solution: hard-wire using ifeq in the local Makefile.