Add c_longdouble

[tgross35]

We don't have all the types needed to actually provide this, so for now this is only a reference. Most of the links in the c_char definition apply here:

• f64 on Windows https://learn.microsoft.com/en-us/cpp/c-language/type-long-double?view=msvc-170
• f64 on Apple https://developer.apple.com/documentation/xcode/writing-arm64-code-for-apple-platforms#Handle-data-types-and-data-alignment-properly
• f128 on aarch64 https://github.com/ARM-software/abi-aa/blob/2024Q3/aapcs64/aapcs64.rst#arm-c-and-c-language-mappings
• f64 on arm32 https://github.com/ARM-software/abi-aa/blob/2024Q3/aapcs32/aapcs32.rst#arm-c-and-c-language-mappings
• f64 on csky https://github.com/c-sky/csky-doc/blob/9f7121f7d40970ba5cc0f15716da033db2bb9d07/C-SKY_V2_CPU_Applications_Binary_Interface_Standards_Manual.pdf Table 2.2: Mapping of C Fundamental Data Types to the C-SKY
• f64 on hexagon https://docs.qualcomm.com/bundle/publicresource/80-N2040-23_REV_K_Qualcomm_Hexagon_Application_Binary_Interface_User_Guide.pdf Table 3-1 Basic data type representation
• f64 on msp430 https://www.ti.com/lit/an/slaa534a/slaa534a.pdf Table 2-1. Data Sizes for Standard Types
• ppc_f128 or f128 on ppc https://gcc.gnu.org/wiki/Ieee128PowerPC
• f128 on riscv32 and riscv64 https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/draft-20240829-13bfa9f54634cb60d86b9b333e109f077805b4b3/riscv-cc.adoc#cc-type-sizes-and-alignments
• f128 on s390x https://linux.mainframe.blog/zarchitecture-principles-of-operation/
• f32 on xtensa https://loboris.eu/ESP32/Xtensa_lx%20Overview%20handbook.pdf Table 59. Data Types and Alignment
• x87_f80 on x86 https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/low-level-sys-info.tex#L198-199

For everything else, try to match Clang.

Some scratch https://godbolt.org/z/YrYKshMMa

r? @ghost

[mati865]

At least 32-bit (dunno about 64-bit) C windows-gnu toolchains use 80 bits for long double, unless -mlong-double-64 is provided.

[rust-bors]

☔ The latest upstream changes (presumably #154668) made this pull request unmergeable. Please resolve the merge conflicts.