Remove IS_NEON from Makefile

[noloader]

The makefile tries to pre-qualify NEON (for lack of a better term), and sets IS_NEON accordingly. If IS_NEON=1, then we go on to perform test compiles to see if -mfloat-abi=X -mfpu=neon (and friends) actually work. Effectively we are performing a test to see if we should perform another test.

The IS_NEON flag predates our compile time feature tests. It was kind of helpful when we were trying to sort out if a platform and compiler options supported NEON without a compile test. That was an absolute mess and we quickly learned we needed a real compile time feature test (which we now have).

Additionally, Debian and Fedora ARMEL builds are failing because we are misdetecting NEON availability. It looks like we fail to set IS_NEON properly, so we never get into the code paths that set either (1) -mfloat-abi=X -mfpu=neon or (2) -DCRYPTOPP_DISABLE_NEON or -DCRYPTOPP_DISABLE_ASM. Later, the makefile builds a *_simd.cpp and the result is an error that NEON needs to be activated (or disabled).

This commit removes IS_NEON so we immediately move to compile time feature tests.

[noloader]

Cleared at Commit 84ab1f3c66a5.