glibc: cleanup long double support

- Add 128 bit long double support from current glibc and a few headers
  they need
- Move the existing 80 bit float support in a sub directory of generic,
  it is not universal to all archs (see file added in docs/develop/arch).
  Also include some new .h files for x86 that are needed after these
  changes (from newer versions of the glibc).
- Adjust Jamfiles for m68k, x86 and x86_64 to use the 80bit format
- Do not adjust arm jamfiles, it was wrongly using 80bit long double and
  should be fixed to use 64bit instead (which means the double functions
  can be used with aliases)
- Do not adjust powerpc jamfiles, because it uses yet another format and
  we build it without long double support anyways.

Note that I moved only the files that were creating compile errors,
quite likely more of the s_* and e_* files need to be moved to the
specific directories, see glibc list here:

https://sourceware.org/git/?p=glibc.git;a=tree;f=sysdeps/ieee754/ldbl-128
https://sourceware.org/git/?p=glibc.git;a=tree;f=sysdeps/ieee754/ldbl-96

Change-Id: Ic2d8a454ba9a3b99638e4fbb63daf02df0fea403
Reviewed-on: https://review.haiku-os.org/c/1143
Reviewed-by: waddlesplash <[email protected]>

docs/develop/arch/long double.md

@@ -0,0 +1,48 @@
+Notes on long double support
+============================
+
+The "long double" type is different on each architecture. Depending on the
+available hardware and ABI conventions, performance compromises, etc, there
+may be many implementations of it. Here is a summary for our convenience.
+
+128-bit IEEE
+------------
+
+Platforms: Sparc, ARM64, RISC-V
+
+This is the standard long double type from IEEE754. It has 1 sign bit,
+15 exponent bit, and 112 fractional part bits. It is the natural extension
+of the 64bit double.
+
+Sparc specifies this type in their ABI but no implementation actually has
+the instructions, they instead trigger a trap which would software emulate
+them. However, gcc short circuits this by default and calls C library
+support functions directly.
+
+64-bit IEEE
+-----------
+
+Platforms: ARM
+
+This is the same representation as plain "double". ARM uses this for simplicity.
+
+80-bit
+------
+
+Platform: x86, x86\_64, m68k
+
+This intermediate format is used by x86 CPUs internally. It may end up being
+faster than plain double there. It consists of a 64bit fractional part, 15
+exponent bits, and 1 sign bit. This is convenient because the fractional part
+is a relatively easy to handle 64bit number.
+
+m68k uses a similar format, but padded to 96 bits (the extra 16 bits are unused).
+
+double double
+-------------
+
+Platforms: PowerPC?
+
+This is also a 128bit type, but the representation is just two 64bit doubles.
+The value is the sum of the two halves. This format allows faster emulation
+than a "true" 128bit long double, and the precision is almost as good.