Fix bus error in bcftools merge on armhf (32-bit hard-float)

While 32-bit ARM mostly allows unaligned access, under certain
conditions it can produce a bus error on unaligned access to
a float.  To prevent this from happening in bcftools merge,
update code that accesses data via bcf_info_t::vptr and
bcf_fmt_t::p to use uint8_t pointers and the le_to_i* macros
in htslib/hts_endian.h.  As a side-effect, this also makes bcftools
merge work on big-endian platforms should anyone attempt to run
it on one.

Fixes samtools#2036 (test_vcf_merge failures on arm 32-bit with FPU: Bus
error).

Makefile

@@ -248,7 +248,7 @@ vcffilter.o: vcffilter.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_
 vcfgtcheck.o: vcfgtcheck.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(htslib_kbitset_h) $(htslib_hts_os_h) $(htslib_bgzf_h) $(bcftools_h) extsort.h filter.h
 vcfindex.o: vcfindex.c $(htslib_vcf_h) $(htslib_tbx_h) $(htslib_kstring_h) $(htslib_bgzf_h) $(bcftools_h)
 vcfisec.o: vcfisec.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(htslib_hts_os_h) $(bcftools_h) $(filter_h)
-vcfmerge.o: vcfmerge.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(htslib_faidx_h) regidx.h $(bcftools_h) vcmp.h $(htslib_khash_h) $(htslib_kbitset_h)
+vcfmerge.o: vcfmerge.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_vcfutils_h) $(htslib_faidx_h) $(htslib_kbitset_h) $(htslib_hts_endian_h) $(bcftools_h) regidx.h vcmp.h $(htslib_khash_h) $(htslib_kbitset_h)
 vcfnorm.o: vcfnorm.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_faidx_h) $(htslib_khash_str2int_h) $(bcftools_h) rbuf.h abuf.h gff.h regidx.h
 vcfquery.o: vcfquery.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_khash_str2int_h) $(htslib_vcfutils_h) $(bcftools_h) $(filter_h) $(convert_h) $(smpl_ilist_h)
 vcfroh.o: vcfroh.c $(htslib_vcf_h) $(htslib_synced_bcf_reader_h) $(htslib_kstring_h) $(htslib_kseq_h) $(htslib_bgzf_h) $(bcftools_h) HMM.h $(smpl_ilist_h) $(filter_h)