fixes #2419 - renewed discussion in #2112 suggests removal of the option was primarily aimed at non-Android builds

The provided -march flags, especially for ARMv5 and ARMv6 may not
necessarily match the needed ones: for ARMv5, it might be armv5,
armv5te, armv5t, etc. If the wrong one is used, the incorrect toolchain
sysroot can be used in a multilib toolchain.

Therefore, let the user building OpenBLAS pass the appropriate -march
flag.

The other flags, such as -mfpu=vfp or -mfloat-abi=hard are kept, as they
are actually required for the build to proceed (OpenBLAS uses VFP
instructions, and assume an EABIhf ABI).

[Peter: update for v0.2.20]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
[Retrieved from:
https://git.buildroot.net/buildroot/tree/package/openblas/0001-Makefile.arm-remove-march-flags.patch]
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

If ARM abi is not explicitly mentioned on the command line, then set the
arm abi to softfp or hard according to the compiler environment.
This assumes that compiler sets the defines __ARM_PCS and __ARM_PCS_VFP
accordingly.

make ARM_SOFTFP_ABI=1

there is no more requirement for ARMv6 instructions,
and VFP on ARMv5 is uncommon

e.g.
make HOSTCC=gcc CC=arm-linux-androideabi-gcc NO_LAPACK=1 TARGET=ARMV7

In Android NDK, it uses armeabi-v7a-hard ABI.
TARGET_CFLAGS += -mhard-float -D_NDK_MATH_NO_SOFTFP=1
TARGET_LDFLAGS += -Wl,--no-warn-mismatch -lm_hard
For more information, please check hard-float example at
android_ndk/tests/device/hard-float/jni/.