Initial import of libdpf.

Co-authored-by: Cursor <cursoragent@cursor.com>

thirdparty/portable-snippets/endian/README.md

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+# Endianness Detection and Swapping
+
+This header provides a convenient way to detect endianness, and
+perform swapping.  It provides 6 important functions:
+
+```c
+psnip_uint16_t psnip_endian_le16(psnip_uint16_t v);
+psnip_uint32_t psnip_endian_le32(psnip_uint32_t v);
+psnip_uint64_t psnip_endian_le64(psnip_uint64_t v);
+psnip_uint16_t psnip_endian_be16(psnip_uint16_t v);
+psnip_uint32_t psnip_endian_be32(psnip_uint32_t v);
+psnip_uint64_t psnip_endian_be64(psnip_uint64_t v);
+```
+
+These are usually implemented as macros, the prototypes above are
+purely for documentation.
+
+These functions will swap byte ordering to or from the provided type
+**if necessary**.  For example, `psnip_endian_le32` is used to convert
+a 32-bit unsigned integer to/from little-endian.  If the machine is
+little-endian nothing will be done, but if it is big-endian then a
+swap will be performed.  It's a bit like `hton`/`ntoh`, except instead
+of host to/from network order, it's host to/from whatever order you
+want.
+
+If you wish to ignore the result of compile-time detection attempts,
+you may define `PSNIP_ENDIAN_FORCE_RT` prior to including this header
+to force run-time detection.
+
+If you're only interested in detection you can just ignore the
+swapping functions mentioned above.  If endianness could be determined
+at compile-time (i.e., through preprocessor macros),
+`PSNIP_ENDIAN_ORDER` will be defined to either `PSNIP_ENDIAN_LITTLE`
+or `PSNIP_ENDIAN_BIG`.
+
+If you need (or prefer) to fall back on run-time detection,
+`PSNIP_ENDIAN_ORDER_RT` will evaluate to the same values
+(`PSNIP_ENDIAN_LITTLE` or `PSNIP_ENDIAN_BIG`).  Note that the
+"run-time" detection will likely be optimized away by the compiler.
+
+## Dependencies
+
+To maximize portability you should #include the exact-int module
+before including endian.h, but if you don't want to add the extra
+file to your project you can omit it and this module will simply rely
+on <stdint.h>.  As an alternative you may define `psnip_uint16_t`,
+`psnip_uint32_t`, and `psnip_uint64_t` to appropriate values yourself
+before including endian.h.
+
+This module requires the builtin portable-snippet module.  If you do
+not include builtin.h before endian.h, endian.h will automatically
+include "../builtin/builtin.h".  If you include builtin.h manually you
+are free to use whatever directory structure you like.