Update PORTING.md to document zlib-ng memory allocation details. #5149

Job	Run time
Ubuntu GCC	16s
Ubuntu 20.04 GCC	12s
Ubuntu GCC OSB	13s
Ubuntu GCC Compat No Opt	13s
Ubuntu GCC ARM SF	21s
Ubuntu GCC ARM SF Compat No Opt	18s
Ubuntu GCC ARM HF	22s
Ubuntu GCC ARM HF No ACLE	52s
Ubuntu GCC ARM HF No NEON	32s
Ubuntu GCC ARM HF Compat No Opt	1m 0s
Ubuntu GCC AARCH64	23s
Ubuntu GCC AARCH64 No ACLE	22s
Ubuntu GCC AARCH64 No NEON	25s
Ubuntu GCC AARCH64 Compat No Opt	30s
Ubuntu GCC MIPS	26s
Ubuntu GCC MIPS64	20s
Ubuntu GCC PPC	31s
Ubuntu GCC PPC No Power8	26s
Ubuntu GCC PPC64	55s
Ubuntu GCC PPC64LE	27s
Ubuntu GCC S390X	21s
Ubuntu GCC S390X No vectorized CRC32	21s
EL9 GCC S390X DFLTCC	6s
EL9 GCC S390X DFLTCC Compat	6s
Ubuntu Emscripten WASM32	45s
macOS GCC Symbol Prefix	43s
macOS GCC Symbol Prefix (ARM64)	21s
macOS GCC Symbol Prefix & Compat	46s
macOS GCC Symbol Prefix & Compat (ARM64)	24s
macOS GCC	1m 5s
macOS GCC (ARM64)	21s
	14m 23s

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