The arch or cpu has its default fpu features and versions such as fpuv2_sf/fpuv3_sf.
And there is also -mfpu option to specify and override fpu version and features.
For example, C860 has fpuv3_sf/fpuv3_df feature as default, when
-mfpu=fpv2 is given, fpuv3_sf/fpuv3_df is replaced with fpuv2_sf/fpuv2_df.
Add CSKY target toolchains to support csky in linux and elf environment.
It can leverage the basic universal Linux toolchain for linux environment, and only add some compile or link parameters.
For elf environment, add a CSKYToolChain to support compile and link.
Also add some parameters into basic codebase of clang driver.
Differential Revision: https://reviews.llvm.org/D121445
Construct LLVM Support module about CSKY target parser and attribute parser.
It refers CSKY ABIv2 and implementation of GNU binutils and GCC.
https://github.com/c-sky/csky-doc/blob/master/C-SKY_V2_CPU_Applications_Binary_Interface_Standards_Manual.pdf
Now we only support CSKY 800 series cpus and newer cpus in the future undering CSKYv2 ABI specification.
There are 11 archs including ck801, ck802, ck803, ck803s, ck804, ck805, ck807, ck810, ck810v, ck860, ck860v.
Every arch has base extensions, the cpus of that arch family have more extended extensions than base extensions.
We need specify extended extensions for every cpu. Every extension has its enum value, name and related llvm feature string with +/-.
Every enum value represents a bit of uint64_t integer.
Differential Revision: https://reviews.llvm.org/D119917
