Investigate malloc/free invocations

[jserv]

rvsim.c is a minimal RV32I simulator without dynamic memory allocation.
Run:

$ build/rv32emu build/rvsim.elf 

Expected results:

...
00000008: 00b76463 ; bltu a3,a0,8        
0000000c: 00008067 ; jalr x0,ra,0        [branch] x[0] <= 16
pc: 0000004c x8:  00000000 x16: 00000005 x24: 00000000
x1: 0000004c x9:  00000000 x17: 00000004 x25: 00000000
x2: 00000000 x10: 00000054 x18: 00000000 x26: 00000000
x3: 00000000 x11: 00000005 x19: 00000000 x27: 00000000
x4: 00000000 x12: 00000060 x20: 00000000 x28: 00000000
x5: 00000000 x13: 00000068 x21: 00000000 x29: 00000000
x6: 00000000 x14: 00000005 x22: 00000000 x30: 00000000
x7: 00000000 x15: 00000060 x23: 00000000 x31: 00000000

Use uftrace to track the number of malloc invocation:

$ uftrace build/rv32emu build/rvsim.elf | egrep "malloc|calloc" | wc -l

It shows 1813 invocations of malloc and calloc, which should be eliminated for performance consideration.

Related: #94

[jserv]

With commit 0031224, the command uftrace build/rv32emu build/rvsim.elf | egrep "malloc|calloc" | wc -l now yields a count of 2, indicating a significant reduction in memory usage.