LoongArch: Handle fp, lsx, lasx and lbt assembly symbols

Like the other relevant symbols, export some fp, lsx, lasx and lbt
assembly symbols and put the function declarations in header files
rather than source files.

While at it, use "asmlinkage" for the other existing C prototypes
of assembly functions and also do not use the "extern" keyword with
function declarations according to the document coding-style.rst.

Cc: stable@vger.kernel.org # 6.6+
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>